In recent years, a safety support system that uses a camera attached to a moving object such as a vehicle to image an external environment has been widely used. When the moving object goes backward for parking or the like, and the safety support system detects, from an image acquired by the camera and depicting an external environment, the existence of an object (cubic object) that may obstruct the movement, the safety support system may notify that the object exists.
As a method of detecting the existence of a cubic object from an image acquired by the camera and depicting an external environment, a method using an optical flow expressing a motion of an object within a digital image by vectors is known, for example. According to this method, multiple temporarily continuous images depicting an external environment are acquired and the amounts (observation flow amounts) of positional shifts of characteristic points extracted from the images acquired at two times are extracted. If the observation flow amounts are larger than the amounts (virtual road surface flow amounts) of positional shifts of characteristic points on a road surface, it is determined that the corresponding characteristic points are points of a cubic object. The method using the optical flow uses a characteristic in which the amount of a motion of a cubic object is larger than the amount of a motion of an image (hereinafter referred to as road surface pattern) of a pattern painted on a road surface or the like. As related art, Japanese Laid-open Patent Publications Nos. 2006-134035, 2007-334859, 2010-204805, 2010-257138, and the like are disclosed, for example.
The aforementioned method, however, assumes that the road surface is flat. For example, if the road surface includes a slope, the road surface pattern may be erroneously detected as a cubic object. Thus, even if the road surface includes the slope, it is desirable that a cubic object be accurately detected without erroneous detection.